Such a vessel is known from U.S. Pat. No. 4,436,048, which describes a vessel comprising a main deck and a turret moonpool which has a circular side and contains a rotary-powered turret having a circular turret side and a flanged turret top.
In offshore technology, vessels such as Floating Production Storage and Offloading vessels or Floating Storage and Offloading vessels or FLNGs (Floating Liquefied Natural Gas production) are typically moored by a turret mooring system. Turret mooring systems provide a turret mooring structure comprising a turret structure and a bearing arrangement with its support structure mounted on the vessel. The turret structure is anchored to the seabed with anchoring lines. The bearing support structure, provided on a vessel, has a receptacle for receiving the turret structure, such that rotation of the vessel about the turret structure is still possible. In this manner, the vessel can weathervane under influence of wind, waves, currents and/or drifting ice and adopt the position of least resistance with regards to the environment.
In addition to the anchoring lines, the turret structure supports one or more riser lines that are connected to wells on the seabed. Through the turret structure the riser lines are connected with process equipment and/or storage equipment on the vessel, allowing transportation of fluids, by establishing a flow path between the vessel and the subsea well.
The turret mooring structure may be an internal turret mooring structure or an external turret mooring structure.
An internal turret mooring structure is provided inside the hull of the vessel, in a so-called moonpool of the vessel. The moonpool receptacle is formed with an opening at or near the bottom of the vessel, facing downwards.
The turret mooring structure itself is connected to the vessel by the bearing arrangement and its support structure, which provide for rotation of the vessel with respect to the turret structure. The turret structure may be a mooring buoy that can be disconnected and reconnected when needed, thereby providing a disconnectable turret mooring system.
Top Mounted Internal Turrets have a bearing arrangement that relies on axial bogies and radials wheels components to allow the turret-moored vessel to weathervane. Axial bogies are designed to take the axial load transferred from the vessel to the turret. Radial wheels transfer the radial load from the vessel to the turret. Typically, the axial bogies are arranged between the radial wheels and a topside process deck of the vessel. On top of the axial bogies arrangement, a collar deck is provided. In the prior art, either no collar deck is present at all, or the axial bogies and radial wheels are positioned in such a manner that the collar deck is vertically close or adjacent to the topside process deck. The top side process deck is typically the deck of the vessel that lies just above the vessel's main deck, and generally holds processing modules used for oil separation, gas in and/or cleaning water before reinjection of the water into the well.
It is observed that due to upscaling in many offshore vessels the elevation of the topside process deck and of the bearing arrangement with respect to the keel has become large.
As FPSO/FLNG vessels have become larger, the elevation of the topside process deck above keel has increased to 40 meter or more. This has a direct effect on the loading of the bearing arrangement, i.e., an increase of the bending forces and moments exerted thereon. Thus such a bearing arrangement requires reinforcement of the axial bogies, resulting in larger and heavier designs and constructions. Such adaptations comprise the implementation of a turret cylinder and a bogie support structure up to the elevation of the topside process deck which implies that additional structures are required to cope with the large height of the vessel.
It is an object of the present invention to overcome or mitigate one or more of the disadvantages from the prior art.